Coherent optical orthogonal frequency division multiplexing (OFDM) and Nyquist wavelength division multiplexing (WDM) are conventionally known by which optical signals are densely arranged in intervals of frequency (wavelength) (see, e.g., Xingwen Yi, “Tb/s Coherent Optical OFDM Systems Enabled by Optical Frequency Combs”, Journal of Lightwave Technology, Vol. 28, No. 14, Jul. 15, 2010, pp. 2054-2061).
However, according to the conventional techniques, the timings of the optical signals to be multiplexed may deviate from one another due to differences in the length of the optical paths caused by wavelength dispersion, etc. Therefore, when the optical signals are densely arranged in intervals of frequency, a problem arises in that suppression of a cross-talk between optical signals is difficult.